Studies on the properties of tubulin were continued. The reaction of the interaction of podophyllotoxin with the colchicine site was evaluated thermodynamically and with analogues. The two molecules share only the trimethoxyphenyl portion of the binding site whereas the tropolone and lignan moieties are seen by different domains. It was also shown that maytansine interacts with both vinblastine sites but the result of binding to these sites differ from those produced with vinblastine. Membrane tubulin has been further characterized to show that the solubilized protein is competent to polymerize with soluble brain tubulin. BIBLIOGRAPHIC REFERENCES: Cortese, F., Bhattacharyya, B. and Wolff, J.: Podophyllotoxin as a probe for the colchicine binding site of tubulin. J. Biol. Chem. 252: 1134-1140, 1977. Bhattacharyya, B. and Wolff, J.: Maytansine binding to the vinblastine sites of tubulin. FEBS Letters 75: 159-162, 1977.